Refrigerant circuits for use in air conditioning systems are well known, and may be of the orifice type, which includes a compressor, a condenser, an orifice, an evaporator and an accumulator, or the expansion valve type, which includes a compressor, a condenser, a receiver dryer, an expansion valve and an evaporator. In these conventional refrigerant circuits if the compressor is started at a time when the refrigerant pressure at the inlet side of the compressor equals the gas pressure at the outlet side, an increase in the drive torque of the compressor results as refrigerant gas flows from the inlet to the outlet, thereby causing a reduction in the rotational frequency of the drive source. This, for example, in the refrigerant circuit for an automotive air conditioning system, reduction of the rotational frequency of the automotive engine may cause torque shock.
Further, in a refrigerant circuit including a compressor with a variable capacity mechanism for uniformly controlling suction pressure, pressure loss increases with increases in passageway resistance between an outlet of the evaporator and an inlet of the compressor in accordance with an increase in the flow rate of refrigerant. Accordingly, refrigerant pressure at the outlet of the evaporator increases responsive to an increase in pressure loss. This then raises the temperature of air which is passed through the evaporator, and reduces the air conditioning capacity.
Because this type of compressor maintains a uniform suction pressure, the temperature of air passing through the evaporator will also be maintained at a constant level. As a result, the air temperature can not be readily controlled in accordance with changes in the automobile atmosphere or the desires of the passengers.